Process for the manufacture of trinitrotoluene

ABSTRACT

Process for preparing TNT comprises nitrating toluene with an anhydrous nitrating agent NO2HSO4 obtained by mixing HNO3 and oleum. The process eliminates pollution problems associated with conventional manufacturing processes and produces TNT in high yields and rapid reaction rates.

United States Patent [191 Hill et al.

[ Mar. 26, 1974 [54] PROCESS FOR THE MANUFACTURE OF 2,475,095 7/1949Hoek 260/645 TRINITROTOLUENE 2,934,571 4/1960 Bonetti 260/645 3,708,5461/1973 Coon et al. 260/645 [75] Inventors: Marion E. Hill, Palo Alto;Wesley E.

w Atherton; Gerald FOREIGN PATENTS OR APPLICATIONS McDonald, Menlo Park,all of Calif. 752,608 7/1956 Great Britain 260/645 1,054,571 1 1967 G B260 645 [73] Assignee: The United States of America as rem mam gr wzg g2: of the Primary Examiner-Leland A. Sebastian g Attorney, Agent, orFirm-Edward J. Kelly; Herbert [22] Filed: Aug. 25, 1972 Berl; A. VictorErkkila [21] Appl. No.1 283,694

[57] ABSTRACT [52 0.5. CI. 260/645, 260/688 Process for Preparing TNTcomprises nitrating toluene [51] Int. Cl. C07c 79/10 with an anhydrousnitraiing agent NOZHSO4 obtained 58] Field of Search 260/645 y mixing Qand oleum- The Process eliminates pollution problems associated withconventional man- 56] R f re Ci ufacturing processes and produces TNT inhigh yields UNITED STATES PATENTS and rapid reaction rates.

2,297,733 10/1942 Wyler et a1. 260/645 14 Claims, 1 Drawing FigureTOLUENE HNO 80 l DNT H 2 50 l 80% TNT DINITRATOR r TRINITRATOR To rWORKUP NITRATING ACID ONT RECYCLE ACIDS SOLVENT I EXTRACTION SURPLUS H2so ro ACID RECOVERY PMENTEDHARZS m4 3799.993

TOLUENE HNO 80 l DNI+ H2 30 80% l TNT DINITRATOR TRINITRATOR WORKUP ANITRATING ACID MT v RECYCLE ACIDS SOLVENT EXTRACTION SURPLUS H 504 TOACID RECOVERY PROCESS FOR THE MANUFACTURE OF TRINITROTOLUENE Theinvention described herein may be manufactured, used and licensed by orfor the Government for governmental purposes without the payment to usof any royalties thereon.

BACKGROUND OF THE INVENTION The invention relates to a process forproducing TNT (2,4,6-trinitrotoluene). In particular the inventionrelates to a process for nitrating toluene to produce TNT.

TNT is conventionally manufactured by a multistep process, whereintoluene is first nitrated with a mixture of nitric acid and sulfuricacid to produce MNT (mononitrotoluene), the MNT is then nitrated to DNT(dinitrotoluene) and the DNT is finally nitrated to TNT, the acidstrength and temperature being increased for each nitro groupintroduced. The TNT obtained contains small amounts of other TNT isomersin addition to the desired 2,4,6-isomer as well as DNT isomers, whichmust be removed in order to produce a TNT of sufficient purity for useas a military explosive. Such purification is usually accomplished by aselliting process, which involves treating the crude TNT with aqueoussodium bisulfite solution, which selectively extacts the undesired TNTisomers from the desired 2,4,6-isomer. The waste liquors resulting fromsuch selliting operations, called red water," cannot be economicallypurified without creating a serious pollution problem. In addition,large amounts of nitrogen oxide fumes are evolved from the nitrationreactors and create a serious atmospheric pollution problem, which iscostly to alleviate.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a process for nitrating toluene to TNT in high yields and rapidreaction rates.

Another object is to provide a novel process for nitrating DNT to TNT inhigh yields.

A further object is to provide a process for nitrating toluene to TNT,which produces TNT of military specification grade, eliminates thepolluting nitrogen oxide fumes emanated from the conventional nitrationreactions and eliminates the sellite process with its attendant redwater disposal problem.

A still further object is to provide a process for nitrating toluenewhich minimizes the formation of metal isomers, maximizes the ratio of2,4- to 2,6-DNT isomers produced and maximizes the yield of desired2,4,6- TNT isomer.

Another object is to provide a continuous, closed loop process fornitrating toluene to TNT.

These and other objects and advantages are achieved by the process ofthis invention, which is based on the use of an extremely activeanhydrous nitrating mixture prepared from HNO and oleum. In accordancewith the process ofthis invention, toluene is nitrated to DNT byreacting it at temperatures below about 40 C with an anhydrous mixtureof I-INO and oleum, and the DNT thus produced is nitrated to TNT,without isolation from the dinitration reaction mixture, by treatment ata temperature of at least about 70 C with an anhydrous mixture of l-INOand oleum. When the trinitration reaction is complete, the reactionmixture is cooled to precipitate the TNT, which is separated, as

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a schematic flowdiagram of a continuous closed-loop process for manufacturing TNT fromtoluene according to this invention.

oEscRiRrIoN 0? THE PREFERRED EMBODIMENTS We consider that the highreactivity of the anhydrous nitrating mixture of HNO and oleum is due inpart to the formation of nitronium bisulfate (NO HSO from HNO3 and S031The stoichiometric conversion of HNO to this nitronium salt provides ahigh concentration of nitronium ion, which is recognized as the reactivespecies in most aromatic nitration reactions. The favorable solvationeffect provided by anhydrous H 80 also contributes to the highreactivity of the system.

US. Pat. No. 2934571 discloses a process for producing DNT substantiallyinstantaneously and in nearly quantitative yield by contacting toluenewith a mixture of l-INO and oleum at a temperature between 50 C and 60C. However, the process of this patent is limited to the production ofDNT and other aromatic compounds containing not more than 2 nitrogroups.

We have now found that TNT can be obtained in over 9Q% yield and rapidreaction rate by nitrating DNT with a mixture of HNO and oleum at atemperature of about C or higher. We have also unexpectedly found thatthe nitration of toluene in the aforesaid manner produces a DNTcontaining less than 2% of meta isomers, which is only half as much asin the current TNT process of manufacture; and that by nitrating the DNTfurther with a nitrating mixture of l-INO and oleum, a TNT product canbe directly obtained having a purity sufficient for military use.

In carrying out the nitration of toluene to DNT with an anhydrousmixture of HNO; and oleum according to the present invention, two molesof l-INO are theoretically required to introduce two nitro groups intoeach mole of toluene; however, it is preferable to employ a small molarexcess of I-INO e.g. about 5%, over the theoretical amount required soas to maximize the yield of DNT produced. Considerably higher molarratios of HNOg to toluene, e.g. 3 to 8 moles of l-INO per mole oftoluene, can be employed to accomplish the dinitration reaction at thelow temperatures employed in the present process, but provide notechnical advantage. Further, it is preferable to employ a small, e.g.about a 10% molar excess of I-INO relative to free S0 since an excess ofSO; over HNO is undesirable due to the competing sulfonation reactionwith toluene. The ratio of free 30;, to H 80, in the oleum employed isrelatively less important and is conveniently about I part by weight per4 parts H The nitration of toluene to DNT can be carried out at minus40-50 C or lower; but it is preferably carried out at a temperature notexceeding about +l C and especially at between about l0 C and 20 C. Atthese preferred temperatures the dinitration is rapid and the productionof undesired meta isomers is minimized. Reaction temperatures aboveabout 40 C. are undesirable, since they promote the formation ofundesired meta isomers.

After the dinitration reaction is complete, the reaction mixturecontaining the DNT and spent acid liquor can be charged with fresh HNO;and S0 as required, and the resulting mixture is heated to a temperatureof about 70 C or higher, preferably between 70 and 100 C., andespecially about 90 C., to effect the nitration of DNT to TNT. Theamount of HNO is sufficient to provide at least one mole of ljNO permole of DNT, as theoretically required. The amount of S0 is preferablyabout one mole, e.g. from 0.9 to 1.1 moles of S0 per mole of HNO Theratio of SO to H 50 employed in the trinitration reaction is notcritical, since sulfonation is not a competing reaction in the nitrationof DNT; it is conveniently about 0.43 to l by weight, which correspondsto oleum containing approximately 30% free S0 To ensure a rapid andcomplete nitration of DNT to TNT, an excess of HNO and S0 is employed,preferably from about 3 to about 5 moles each of l-lNO and 50;, per moleof DNT. The use of excess reagents in the trinitration step does notimpair the economy of the total process for TNT manufacture, since theexcess HNO and S0 can be employed in the dinitration step of aclosed-loop process. Preferably, in a closed-loop process the amounts ofHNO and S0 added for the trinitration step are such that the acidliquor, recovered by filtration of the trinitration reaction mixture toremove TNT, has approximately the composition of the HNO,,-oleumnitration mixture required for the dinitration step.

In the process of the invention additional amounts of sulfuric acid areSO by reaction of free S03 with the water formed in the nitrationreactions. The production of such additional amounts of sulfuric acidresults in an increase in both the volume and ratio of H50, to $0 whichis undesirable in closedloop operation. To prevent such build-up ofsulfuric acid in a closed-loop process, a sufficient portion of thereaction mixture obtained in the dinitration step is withdrawn and theDNT fontent is separated therefrom, e.g. by extraction with a suitableinert organic solvent, e.g. methylene dichloride, which is thenseparated from the DNT by distillation. The DNT thus recovered isreturned to said reaction mixture for the trinitration step and the acidliquor remaining after removal of its content of DNT is disposed orutilized in suitable manner.

We have found that the reaction mass obtained in the trinitration steppossesses a high capacity for retaining the unsymmetrical TNT isomersand other by-products and DNT in solution, with the result that TNT ofexcellent purity can be obtained directly from the cooled reaction massby filtration followed by washing with water to remove adherent acid.The TNT crystals recovered from the reaction mass can be furtherpurified by washing with sulfuric acid or nitric acid of a concentrationat least about 50%, or mixtures of such acids.

The following example specifically illustrates the process of thisinvention.

EXAMPLE Part A. Preparation of Dinitrotoluene (DNT) A nitrating mixtureof 25.1 g (398 mmoles) of absolute nitric acid and 143.9 g of 20% oleumconsisting of 29.6 g (370 mmoles) free 80;, and I 14.3 g H 80, wasplaced in a two-piece glass reactor provided with a thermometer and aglass agitator shaft with a Teflon blade. The bottom part of the reactorwas a 100 ml flask, which was joined to the top by a ground glass flangeand was fitted with an internal glass coil for heating the contents withsteam or cooling with water; and the top part was fitted with athermometer port and three other ground glass joints. l 7.3 g (188mmoles) of toluene (refined grade) were added dropwise during 100minutes to the agitated nitrating mixture which was maintained at about-8 to -l0 C., after which the reaction mixture was agitated for '15minutes at -8 to lO C. and then allowed to warm to room temperature.

A sample (18.2g) of the solution thus obtained was removed, poured onice and the DNT was separated. A yield of 99.4% (based on toluene) ofDNT having the following composition (by glc analysis) was obtained:

2.6- DNT 15.6% 2,3- and 2,5- DNT 0.6% 2,4- DNT 82.6% 3,4- DNT 1.2%

Thetotal meta isomers in the DNT thus obtained was 1.8%. Part B.Preparation of TNT The solution remaining in the reactor after removalof the 18.2g sample in part A was fortified by adding 60.1 g. mmoles) of(a stabilized SO marketed under the name Sulfan by Allied ChemicalCorp.) and 46.1 g. (73l mmoles) of absolute HNO in that order with icebath cooling from dropping funnels equipped with Teflon stopcocks. Thetotal amounts of S0 and HNO in the resulting solution were 75] mmoleseach, allowing for the HNO remaining after the dinitration step and alsothe amount removed in the 18.2 g. aliquot; and the amount of DNTremaining in the reactor was 171 mmoles. Trinitration was effected byheating the resulting solution at 70 C. for 30 minutes, then at 80 C.for 15 minutes and finally at C. for 15 minutes. This was accomplishedby placing an oil bath preheated to 70 C. around the flask andintroducing steam through the reactor coil to heat the solution to 70 Cwithin 1 minute, and maintaining the temperature within i 2 C. byintermittent flow of a small amount of cooling water. The temperaturewas raised to 80 C. in 40 seconds and finally from 80 to 90 C. in 45seconds. The hot reaction mixture was poured into an Erlenmeyer flaskand'cooled to 0 C. and the TNT crystals thus obtained were separated byfiltration on a coarse, sintered glass funnel. The mother liquor waschilled to l0 C. and filtered to obtain a second crop of TNT crystals ona separate filter. The crystal crops were combined, washed with wateruntil acid free and dried. The dried crystals had a setting point of 802C. and weighed 36 grams, corresponding to a yield of 94% of theory basedon the toluene starting material. (A product of somewhat higher settingpoint was obtained by washing the filter cake with 70% H 80 prior towashing with water as described below).

About 5% of the DNT was oxidized and the remaining 95% was converted toTNT. About 1% of the TNT was subsequently oxidized, resulting in a netyield of about 94%. The products of the DNT oxidation were CO CO, N andH 0 while the TNT was converted, at least in part, to severalby-products, mainly 2,4,6- trinitrobenzoic acid.

RECYCLE CLOSED-LOOP OPERATION The acid filtrate, obtained by removingthe TNT crystals from the reactor mass in Part B above had very nearlythe desired composition for the dinitration reaction, as described inpart A above, and its composition was adjusted, as required, by additionof a small amount of l-lNO orO to provide the aforesaid rago of 398mmoles HNOQW mmoles 8?} per 188 mmoles toluene as employed in part Aabove. The cycle was then repeated by adding 17.3 g. (188 mmoles) oftoluene and conducting the dinitration and trinitration reactions asdescribed in part A and part B above, respectively. For fortification ofthe reaction mass resulting from the dinitration reaction, the amount ofDNT formed, the amount of HNO remaining and the amount of H SO producedwere calculated on the assumption of a l 00 yield of ljNT according tothe equation:

and adding the necessary amounts of S0 and HNO to adjust the acids fortrinitration as described in part 8.

Nine cycles were carried out in the foregoing manner. The composition ofthe acid phases used for the dinitration and trinitration reactions inthree of the cycles is shown in the following table:

ACID PHASE COMPOSITION Dinitration Trinitration The dinitration reactionin the first cycle was carried out with fresh acids so that neithernitrosyl sulfuric acid (NOHSOJ nor dissolved organics were present.NOHSO, was formed during each of the trinitration reactions due to thecompeting oxidation reaction. However, the buildup of NOHSO, wascompensated by removing a portion of the filtrate after eachtrinitration reaction for analysis.

The NOHSO, concentration was lower in the trinitration step than in thedinitration step due to dilution by the S0 and HNO added to fortify theacid phase. The dissolved organics present in the acid phase at thestart of the dinitration reaction were the impurities and TNT that hadnot crystallized out following the previous trinitration reaction; thesematerials plus DNT constitutecl the organics dissolved in the acid phaseat the start of the trinitration reaction. The columns headed by givethe relative weight percentage composition of the acids exclusive of thedissolved organics.

The average yield of TNT baed on toluene was 93% of theory for the ninecycles. This figure included the TNT that crystallized out of the acidphase, together with the impure TNT that remained in solution, (2,4,6-TNT was the major component of the organic materials in the acid phaseafter crystallization of the TNT; the total amount of dissolved organicswas regarded simply as impure TNT). The method of operation did notallow determination of a value for the yield of crystallized TNT due toloss of a portion of the dissolved TNT in the by-product acid. Thequality of the TNT obtained in the nine cycles of operation wasconsistently good, indicating that the acid phase possessed a highcapacity for retaining the unsymmetrical TNT isomers and otherby-products.

PURIFYING TNT The wet filter cake of TNT crystals obtained by separatingthe crystals from the nitration mixture in the last cycle, was slurriedin about an equal weight of H (corresponding to about 1.5 g. of 70% H SOper gram of TNT solids) at room temperature. The slurry was filtered andthe filter cake was washed acidfree with water and dried. The purifiedTNT crystals thus obtained had a setting point of 803 C., which issatisfactory for type I military explosive-grade TNT. The foregoingdemonstrates that the novel process is capable of producing TNT ofacceptable military explosive grade without the use of the conventionalsellite purification process.

The present process can be readily adapted for continuous manufacture ofTNT. The drawing shows a schematic flow diagram of a continuousclosed-loop process for production of TNT from toluene according to theprocess of the present invention. Metered streams of toluene and freshor recycled nitrating acid, in the ratio of 2.1 moles of HNO 2 molesofSO and about 3-4 moles of H SO per mole of toluene, are fedcontinuously to the dinitrator which is operated at about l0 C. Freshnitrating acid is used for startup; thereafter recycled nitrating acidfrom the trinitrator is employed (see below). The dinitrator can be astainless steel tank provided with an agitator, cooling coils, bottomfeed ports and top outlet for effluent reaction mixture. The averageresidence time of the reactants in the dinitrator is about 30 minutes,which as noted above is substantially longer than the time required tocomplete the nitration of toluene to DNT under these conditions. Theeffluent from the nitrator contaiing the DNT in H SO and little if anyl-lNO or NO HSO is divided such that, for example, 80% thereof flowsdirectly to the trinitrator and 20% flows to a solvent extraction unit,such as a Scheibel extractor, wherein the DNT is continuously extractedfrom the acid liquor with a suitable solvent, e.g. methylene chloride.The surplus acid from the extractor is pumped to storage for recovery orsale and the DNT extract flows to a still (not shown), where the solventis separated by distillation, and the DNT thus obtained is fed to thetrinitrator. Additional l-lNO and S0 are continuously fed into thetrinitrator along with the eftluent from the dinitrator so as toprovideappr oximately 3 moles of NO2HSO4 (3 moles each of l-lNO and S0per mole of DNT. The trinitrator, which can be a vessel similar to thedinitrator but provided with heating coils, is operated at about C.

with an average residence time of reactants of about 1 hour, which issufficient to complete the conversion of DNT to TNT under theseconditions. Instead ofa single trinitration vessel, the reaction mixturecan be fed, for example, through a series of three trinitrators, whereinthe first is maintained at 70 C., the second at 80 C. and the third at90 C. and the residence time in each is suitably adjusted, asillustrated in the foregoing example. The reaction mixture from thetrinitrator flows to a workup installation, where it is cooled to aboutlO C. to crystallize the TNT, and the TNT crystals are filtered off andpurified, if necessary, by suitable means, e.g. washing with 70% H 80then washed with water and dried. The mother liquors separated from thecrystallized TNT are of essentially the required concentration toconvert the toluene feed to DNT and are recycled to the dinitrator,together with additional HNO and S if required.

It is thus evident that the process of this invention provides animportant advance in the manufacture of TNT, including the followingspecific advantages:

1. It provides rapid nitration reaction rates and high yields of TNT.

2. It produces TNT of military specification grade directly bycrystallization from the trinitration reaction mixture followed bywashing with water. If necessary, the TNT can be further purified bywashing with conc. H 80 3. It obviates the sellite process currentlyused to purify crude TNT and eliminates the red water disposal problemassociated therewith, which is a major pollution problem that plaguesthe current method for TNT manufacture.

4. It does not produce N0 fumes as by-products because both NO HSO andNOT- S0 are thermally stable in the anhydrous HNO oleum system used inboth the diand trinitration reactions, thereby eliminating theatmospheric pollution due to N0 fumes evolved in the current process forTNT.

We wish it to be understood that we do not desire to be limited to theexact method and detail of construction described for obviousmodification wll occur to persons skilled in the art.

What is claimed is:

l. A process for producing TNT which comprises contacting toluene withan anhydrous nitrating mixture of HNO and oleum at a temperature below40 C to produce DNT, heating the DNT in situ at a temperature of atleast about 70 C with an anhydrous nitrating mixture of HNO and oleum toproduce TNT, and recovering the TNT from the reaction mixture, whereinthe amount of HNO is at least 2 moles per mole of toluene and the amountof free S0 in the oleum does not exceed 1 mole per mole of HNO in thenitration of toluene to DNT, and the amount of HNO is between 1 and 5moles per mole of DNT in the nitration of DNT to TNT.

2. The process according to claim ll, wherein the toluene is nitrated ata temperature between about 20 C. and C. and the DNT is nitrated at atemperature between about 70 C. and 100 C.

3. The process according to claim 1, wherein the amount of HNO isbetween about 3 and 5 moles per mole of DNT and the amount of free SO;,in the oleum is about 1 mole per mole of HNO in the nitration of DNT.

4. The process according to claim 3, wherein the amount of HNO; isbetween about 2.l and 2.2 moles per mole of toluene.

S. The process according to claim 1, wherein the reaction mixture fromwhich the TNT is recovered is recycled for use in the nitration oftoluene.

6. The process according to claim 5, wherein a portion of the reactionmixture obtained by nitrating toluene to DNT, is contacted with aninert, immiscible liquid organic solvent for DNT, the resulting solventsolution of DNT is separated from the acid liquor, and the DNT isrecovered from the solvent solution and submitted along with theremainder of the reaction mixture to the nitration of DNT to TNT.

7. The process according to claim 6, wherein the organic solvent ismethylene chloride.

8. The process according to claim 1, wherein the TNT separated from thereaction mixture is washed with an acid of the group consisting of50-100% H 50, and 50-100% HNO and mixtures thereof.

9. A continuous process for producing TNT, which comprises feeding intoa first reaction vessel toluene and an anhydrous nitrating mixture ofHNO and oleum wherein the amount of l-INO is at least 2 moles per moleof toluene and the amount of free 50;, in the oleum does not exceed 1mole per mole of HNO maintaining the reactants in said vessel at atemperature below 40 C. to product DNT, passing the reaction mixturefrom said first reaction vessel into a second reaction vessel togetherwith sufficient anhydrous nitrating mixture of HNO and oleum to providea ratio of between 1 and 5 moles of HNO per mole of DNT, maintainingsaid mixture in the second vessel at a temperature of at least about C.to product TNT, separating the TNT from the reaction mixture, andrecycling the recovered reaction mixture to the first reaction vessel.

10. The process according to claim 9, wherein a portion of the reactionmixture from the first reaction vessel is contacted with an inert,immiscible liquid organic solvent for the DNT, the resulting solventsolution of DNT is separated from said reaction mixture, and the DNT isrecovered from said solvent solution and introduced along with theremainder of the reaction mixture to the second reaction vessel.

11. The process according to claim 10, wherein the organic solvent ismethylene chloride.

12. The process according to claim 10, wherein the amount of HNO isbetween about 2.1 and 2.2 moles per mole of toluene and the amount offree 50;, in the oleum is less than 1 mole per mole of HNO in the firstreaction vessel, and the amount of HNO is between about 3 and 5 molesper mole of DNT and the amount of free in the oleum is about 1 mole permole of HNO in the second reaction vessel.

13. The process according to claim 12, wherein the oleum in the firstreaction vessel contains about 20% by weight of free $0 and the oleum inthe second reaction vessel contains about 30% by weight of free $0 14.The process according to claim 12, wherein the first reaction vessel isoperated at a temperature between about 20 C. and +10 C. and the secondreaction vessel is operated at a temperature between about 70 C. and C.

2. The process according to claim 1, wherein the toluene is nitrated ata temperature between about -20* C. and +10* C. and the DNT is nitratedat a temperature between about 70* C. and 100* C.
 3. The processaccording to claim 1, wherein the amount of HNO3 is between about 3 and5 moles per mole of DNT and the amount of free SO3 in the oleum is about1 mole per mole of HNO3 in the nitration of DNT.
 4. The processaccording to claim 3, wherein the amount of HNO3 is between about 2.1and 2.2 moles per mole of toluene.
 5. The process according to claim 1,wherein the reaction mixture from which the TNT is recovered is recycledfor use in the nitration of toluene.
 6. The process according to claim5, wherein a portion of the reaction mixture obtained by nitratingtoluene to DNT, is contacted with an inert, immiscible liquid organicsolvent for DNT, the resulting solvent solution of DNT is separated fromthe acid liquor, and the DNT is recovered from the solvent solution andsubmitted along with the remainder of the reaction mixture to thenitration of DNT to TNT.
 7. The process according to claim 6, whereinthe organic solvent is methylene chloride.
 8. The process according toclaim 1, wherein the TNT separated from the reaction mixture is washedwith an acid of the group consisting of 50-100% H2SO4 and 50-100% HNO3and mixtures thereof.
 9. A continuous process for producing TNT, whichcomprises feeding into a first reaction vessel toluene and an anhydrousnitrating mixture of HNO3 and oLeum wherein the amount of HNO3 is atleast 2 moles per mole of toluene and the amount of free SO3 in theoleum does not exceed 1 mole per mole of HNO3, maintaining the reactantsin said vessel at a temperature below 40* C. to product DNT, passing thereaction mixture from said first reaction vessel into a second reactionvessel together with sufficient anhydrous nitrating mixture of HNO3 andoleum to provide a ratio of between 1 and 5 moles of HNO3 per mole ofDNT, maintaining said mixture in the second vessel at a temperature ofat least about 70* C. to product TNT, separating the TNT from thereaction mixture, and recycling the recovered reaction mixture to thefirst reaction vessel.
 10. The process according to claim 9, wherein aportion of the reaction mixture from the first reaction vessel iscontacted with an inert, immiscible liquid organic solvent for the DNT,the resulting solvent solution of DNT is separated from said reactionmixture, and the DNT is recovered from said solvent solution andintroduced along with the remainder of the reaction mixture to thesecond reaction vessel.
 11. The process according to claim 10, whereinthe organic solvent is methylene chloride.
 12. The process according toclaim 10, wherein the amount of HNO3 is between about 2.1 and 2.2 molesper mole of toluene and the amount of free SO3 in the oleum is less than1 mole per mole of HNO3 in the first reaction vessel, and the amount ofHNO3 is between about 3 and 5 moles per mole of DNT and the amount offree SO3 in the oleum is about 1 mole per mole of HNO3 in the secondreaction vessel.
 13. The process according to claim 12, wherein theoleum in the first reaction vessel contains about 20% by weight of freeSO3 and the oleum in the second reaction vessel contains about 30% byweight of free SO3.
 14. The process according to claim 12, wherein thefirst reaction vessel is operated at a temperature between about -20* C.and +10* C. and the second reaction vessel is operated at a temperaturebetween about 70* C. and 100* C.